1. Field of the Invention
The invention disclosed and claimed herein generally pertains to an apparatus and method for using a local constraint to enhance attachment of an IC package, chip or other device to a PCB or other mounting platform, when a BGA or the like is used for the attachment. More particularly, the invention pertains to an apparatus and method of the above type which eliminates the need to underfill the IC package, chip or device, after it has been soldered to the PCB or other platform.
2. Description of the Related Art
It has become quite common to use ball grid arrays (BGAs) and micro BGAs (C4s) to attach IC packages, chips and other IC devices to printed circuit boards (PCBs) or other platforms. A BGA comprises multiple balls of solder, which are typically joined to electrically conductive pads or terminals along a planar surface of the IC package or other device. The IC device and BGA are placed onto a PCB, so that respective solder balls of the BGA are in contact with corresponding conductive pads of the PCB substrate. This configuration is then heated, such as by means of a reflow oven, so that the solder balls melt. Thereafter, the solder is cooled and solidifies, and thus bonds the package or device to the PCB.
Notwithstanding the benefits of BGAs for surface mount attachment of IC devices, as described above, this means of attachment also has a significant drawback. Because the soldering process requires a heating phase followed by a cooling phase, both the IC device and the PCB substrate experience thermal expansion and thermal contraction. However, it is common for the coefficient of thermal expansion (CTE) of the IC device attached by the BGA to be different from the CTE of the PCB substrate, and to typically be quite less. This difference can cause the solder balls to deform, and may stress the solder joints to such an extent that the joints break or fracture.
At present, this problem is addressed by means of a practice referred to as underfilling. Underfilling is used, for example, in connection with IC devices that are high performance, high I/O count devices. In an underfilling process, after a BGA has been used to solder an IC device to a PCB as described above, an underfill (which could be an epoxy) mixture is injected between the IC device and the PCB. The device is thus glued to the PCB surface and rigidly held in place with respect thereto. However, a serious disadvantage with refilling is that if it becomes necessary to rework the BGA connections, any such rework effort will be difficult or impossible.